ext4: use memtostr_pad() for s_volume_name

In the Linux kernel, the following vulnerability has been resolved:

ext4: use memtostr_pad() for s_volume_name

As with the other strings in struct ext4_super_block, s_volume_name is

not NUL terminated. The other strings were marked in commit 072ebb3bffe6

("ext4: add nonstring annotations to ext4.h"). Using strscpy() isn't

the right replacement for strncpy(); it should use memtostr_pad()

instead.